This invention relates generally to speed control of electric motors, and, more particularly, to an electronic system for controlling the speed of a shaded-pole motor.
There are many applications for variable speed where brush or universal motors are unsuitable because of their high cost and objectionable mechanical noise. Shaded-pole motors have long been used in tape recorders, phonographs, desk fans, air circulators and the like where their simplicity, quiet operation, low cost and low maintenance are well suited to the requirements. The inherently high slip of a shaded-pole motor makes it convenient to obtain speed variation by changing the impressed voltage. For example, it is common practice to provide multi-speed operation in such high-load applications as fans by incorporating a small series reactor in the fan base, which can be switched in or out at will. However, this kind of control, while providing multi-speed operation, does not have the variable speed control necessary for many applications where the load is light in nature, among them the production of pulsato for musical instruments. An example of apparatus for producing pulsato by acoustic devices is described in Leslie U.S. Pat. No. 3,245,284 which discloses a main motor for driving a rotor at full pulsato speed, and a secondary motor having an axially floating rotor that is spring-biased normally to be out of alignment with its stator, which when energized pulls the floating rotor into alignment with the stator by solenoid action. This movement is utilized to provide a releasable frictional coupling between the shaft of the secondary motor and a friction wheel carried on the main motor shaft. When the secondary motor is energized and the main motor de-energized, the acoustic rotor is driven at the slower speed of the secondary motor. For producing the "fast", or full pulsato speed, the acoustic rotor is rotated at about five to eight revolutions per second, and for "slow" operation the rotational speed of the rotor is about one-half to one revolution per second. A disadvantage of this sytem is that when the main drive motor is turned off following operation at full pulsato speed, the rotor slowly coasts to a stop without any braking effect, and produces undesirable droning sounds. This is overcome by the system described in Leslie's Patent Ser. No. 4,198,880, which also uses two motors but which provides automatic braking from full pulsato speed to either "slow" speed or to a completely stopped condition. The motors employed in both systems are preferably of the shaded-pole type, but the fact remains that two motors are required to provide "slow" and "fast" operation.
Pulsato producing systems are also known in which an acoustic rotor is belt-driven by a single drive motor and adjustment of pulsato rate between "fast" and "slow" is accomplished with multi-step pulleys. Such systems suffer the disadvantage that different sets of pulleys are required for different line current frequencies; that is, larger pulleys are required in countries where 50 Hz is the standard line frequency in order to obtain the proper pulsato rate, thus frustrating efforts to design a universal pulsato system.
In addition to the series reactor control mentioned earlier, shaded-pole motors have heretofore been controlled by changing the impressed voltage in response to shaft speed information derived by means of a tachometer. This prior art system suffers the serious defect of taking a long time to slow down; thus, if used in an acoustic pulsato system, the earlier-mentioned undesirable droning sounds would be produced when going from "fast" to "slow" operation, or from "slow" to stop.
Ideally, an acoustic pulsato system should include a single, simple, inexpensive, quiet motor for belt-driving a rotor. The speed of the motor should be capable of precise control over approximately a ten-to-one range of both "fast" and "slow" operation, respectively. In addition, the motor should be automatically braked when it is desired to change from "fast" to "slow" or from "slow" to "stop" operation. Other desideratum are that the braking current be automatically removed when the motor shaft has either reached its new operating speed or when the shaft is stopped after motor turn-off, that the motor speed up rapidly when power is applied, and that the speed control be essentially immune to variations in line voltage and frequency.